Automobiles which can operate on alternative fuels, such as arbitrary mixtures of alcohol and gasoline, are indicative of future trends and, in fact, will soon be required by law in certain regions. For proper engine operation it will be necessary to measure the ratio of alcohol-to-gasoline within the fuel mixture which is being injected into the combustion chamber. Since the automobile may be filled with gasoline at one instance and an alcohol-containing gasoline mixture of up to about 85% methanol at the next, this ratio is likely to change between fill-ups. Further, because alcohol and gasoline can physically separate in the gas tank, this ratio may continuously vary during the operation of the automobile. Therefore, it is necessary that this ratio be continuously monitored to assure proper operation of the automobile's engine.
A variety of techniques have been previously proposed for making these on-board measurements of the alcohol content within the fuel mixture for control of the engine parameters. Typically, these methods have measured various properties of the gasoline mixture, including the dielectric constant, thermal conductivity, index of refraction, change in the speed of sound through the mixture and microwave absorption. However, these methods tend to be prohibitively expensive for widespread use or the measuring techniques involved are inherently problematic since they tend to be strongly dependent on temperature and/or the detailed properties of the gasoline used. Further, as an exacerbation of these shortcomings, the composition of a particular gasoline mixture may vary considerably even within a single name brand. Therefore, these methods have failed to provide the reliability required for automotive engine control applications.
An alcohol sensing device based on infrared spectroscopy methods would generally avoid the problems associated with these previous methods, including the strong dependence on temperature and/or the gasoline composition. This is because infrared spectroscopy is an analytical technique which measures the relative absorption of various infrared wavelengths by a particular specimen and is thereby dependent on the molecular constitution of the specimen. A sensor for determining the alcohol content in gasoline which utilizes such an infra-red absorption technique is disclosed in U.S. Ser. No. 07/699,018 filed May 13, 1991 in the name of Harris et al and assigned to the assignee of the present invention overcomes many of the problems cited above. Harris et al disclose an alcohol sensing device which utilizes infrared spectroscopy measuring techniques capable of detecting alcohols without any interference from the presence or absence of gasoline.
The Harris device contains a single light source which emits a spectrum of light including the near-infrared wavelengths. The sensor measures the ratio of light transmission (or conversely--light absorption by the fuel mixture) at two discrete wavelengths within the near-infrared spectrum. Alcohols will generally absorb different wavelengths of light as compared to alcohol-free gasolines because the alcohols contain oxygen-hydrogen (O--H) bonds while gasolines practically have no O--H bonds.
Therefore, Harris et al teach the selection of two particular wavelengths such that at one of the infrared wavelengths, alcohol is strongly absorbing because of the vibrational overtone transition associated with the O--H bond in the alcohol molecule. At this same wavelength however, the gasoline is more transparent, i.e., exhibits very little absorption, since O--H bonds are almost entirely absent from gasoline. The second wavelength is chosen so that both the alcohol and the gasoline exhibit little absorption of the infrared wavelength and are therefore nearly transparent.
During operation of the Harris device, a beam of light is emitted which contains the two discrete wavelengths within the near infrared spectrum. The light beam is transmitted such that the two discrete wavelengths traverse the same optical path. Two detectors are adjacently disposed so as to receive the emitted light after transmission through the alcohol/gasoline fuel mixture.
The first detector determines the amount of infrared absorbance by the fuel mixture at the first wavelength by filtering all other wavelengths except the first wavelength of interest. Similarly, the second detector determines the amount of infrared absorbance by the fuel mixture at the second wavelength. The two detectors are thermopile detectors which convert the received light into heat. Accordingly each of the two thermopile detectors generates an increase in temperature corresponding to the amount of transmitted light received at the two particular wavelengths. The temperature increases are then converted by thermopiles into voltage signals which are readily measured.
Once these signals for the two wavelengths are obtained, the ratio of the amounts of absorption by the alcohol/gasoline fuel mixture at both wavelengths can be computed. However, in order to obtain an adequate signal-to-noise ratio, the light source taught by Harris et al must be alternated between a high power and low power setting. The reason for this is that excessive thermal "noise" would otherwise be fed into the computation of the alcohol-gasoline ratio based upon the light received by the two thermopiles. As an example, the ambient temperature of the thermopiles will likely differ, i.e., drift, with respect to time during the operation of the engine. Because a thermopile converts temperature difference into a voltage output, all thermal sources to which the thermopile is exposed will influence its voltage output, whether the source is the intended light source or the temperature of the substrate to which the thermopile is secured, as will be discussed more fully later.
As a result, though this type of device taught by Harris et al utilizes infrared absorption spectroscopy, it is still strongly dependent on temperature stability due to the nature of its detection system. This dependency has a particularly adverse effect in an environment such as that of automobiles. Within the engine environment of an automobile, temperatures may fluctuate greatly over a wide range from about -40.degree. C. up to about 120.degree. C., making it difficult in practice to maintain the two thermopiles of this device at constant temperatures throughout the operation of the engine. If the ambient temperature drift is not compensated for, the absorbance measurements by the detectors will give erroneous results. Accordingly, these various thermal sources must all be accounted for in the algorithm used to determine the air-fuel ratio in order to ensure an accurate measurement.
With the above conditions in mind, the shortcomings of the Harris device may be more fully explained as follows. The voltage output of a thermopile may be represented by V.sub.o which is the sum of a voltage resulting from the energy absorbed from the light V.sub.s plus a voltage resulting from the ambient temperature influencing the thermopile V.sub.d. By providing a high and low setting for the light source, two values for V.sub.s are obtained -V.sub.sh and V.sub.sl, for the high and low settings, respectively--while V.sub.d remains essentially constant within any given cycle of the light source. The difference between the high and low setting voltage outputs V.sub.oh and V.sub.ol can now be determined as follows: EQU V.sub.oh -V.sub.ol =(V.sub.sh +V.sub.d)-(V.sub.sl +V.sub.d)=V.sub.sh -V.sub.sl
From the above it can be seen that the voltage output due to the ambient temperature of the substrate V.sub.d has been factored out of the equation. What is left is the difference between the voltage due solely to the modulation of the light source, V.sub.sh -V.sub.sl. By taking a ratio between the first and second thermopiles, or (V.sub.sh -V.sub.sl).sub.1 /(V.sub.sh -V.sub.sl).sub.2, an absolute value for the relative absorbances of the fuel at the two frequencies can be ascertained. The concentration of alcohol in the fuel is then determined from standard absorption data.
Though the device taught by Harris et al has been noted to perform well, certain disadvantages associated with the modulation of the light source have been identified. Obviously, the need to modulate the light source between a high and low intensity setting complicates the method of analyzing the fuel ratio, requiring an algorithm which controls the timing and intensities between the high and low settings. An additional disadvantage is that the response time of the sensor is slower as a result of the time required to heat and cool the filaments of the light source between settings so as to avoid errors in the thermopile outputs. As an example, the thermal decay time for the filaments used by Harris et al was approximately 200 milliseconds. This contrasts to the response time of approximately 28 milliseconds for the thermopile sensors themselves. Accordingly, the cycle time for the Harris device was not limited by the capability of the type of sensor used, but by the form of light source used.
Though light sources having substantially shorter thermal decay times may be appropriate under some circumstances, the harsh environment of the automobile severely limits the choices available for sensing the fuel ratio as described. Therefore, it would be desirable to provide an alcohol sensor for determining the alcohol content in a fuel mixture for use in an automobile environment, which utilizes infrared absorption spectroscopy techniques as previously disclosed by Harris et al, but which alleviates the shortcomings associated with the need to modulate the light source between a high and low intensity setting. In particular, it would be desirable to provide such an alcohol sensor which can operate satisfactorily with the light source emitting light at only one intensity while still being able to factor in ambient temperature effects and other extraneous external influences.